Longitudinal Tube Research Articles

Evaluation of elliptical finned-tube heat exchanger performance using CFD and response surface methodology

The elliptical tube has long been considered as an alternative of the circular tube in finned-tube heat exchangers (FTHE). Most of the previous studies evaluated the two tube shapes only based on the air-side performance of FTHE, and did not consider any interaction effect of the axis ratio with other parameters. We proposed a numerical model to simultaneously predict the fluid flow and heat transfer on both air-side and water-side of elliptical FTHE. The numerical results agree well with the reported experimental data. Response surface methodology was applied to understand the interactions among seven design factors including number of rows, axis ratio, transversal tube pitch, longitudinal tube pitch, fin pitch, air velocity, water volumetric flow rate. Response surface analysis was used to evaluate the axis ratio effect on the overall thermal–hydraulic performance which was quantified by the heat transfer rate per unit power consumption. The results indicate that the axis ratio strongly interacts with air velocity and water volumetric flow rate. The increase of axis ratio improves the overall thermal–hydraulic performance at higher air velocity or lower water volumetric flow rate, but the opposite effect is observed at lower air velocity or higher water volumetric flow rate.

English

This paper presents the numerical study on the two-dimensional forced convection heat transfer for staggered tube banks in cross flow under incompressible, steady-state conditions. This system is solved on the body fitted coordinates (BFC) using the finite volume method (FVM) for the flow over a bundle of cylindrical tubes. The constant heat flux is imposed on the surface of the tubes as the thermal boundary condition. The type of the arrangement is considered a staggered of tubes. The longitudinal pitch to tube diameter ratios (ST/D) of 1.25, 1.5 and 2 are also considered. Reynolds numbers are varied from 25 to 250 and Prandtl number is taken as 0.71. Velocity field vectors, temperature contours and local and average Nusselt numbers were analysed in this paper. It can be seen that the predicted results are good agreements with previous experimental and numerical results. The obtained results show that the heat transfer rate increases with decreases of the step to the longitudinal tube diameter. The local heat transfer strongly depends on the Reynolds number. It tends to obtain the highest values at the surface opposite to the direction of flow. The heat transfer rate is insignificant in the areas of recycling.

Parametric study and field synergy principle analysis of H-type finned tube bank with 10 rows

In this paper, three-dimensional numerical studies are performed for heat transfer and pressure drop characteristics of H-type finned tube bank with 10 rows by software FLUENT. The effects of seven geometric parameters (tube row number, fin thickness, slit width, fin height, fin pitch, spanwise tube pitch and longitudinal tube pitch) and Reynolds number are examined. It is found that for the tube bundle studied, the heat transfer and fluid flow are in the developing region, and they become periodically fully developed after the 10th row. Among the seven geometric parameters spanwise tube pitch has the most important effect and slit width has the least important effect, with other five parameters in between. The results are also analyzed from the view point of field synergy principle. It is found that the effects of the eight parameters on heat transfer and fluid flow characteristics can be well described by the field synergy principle. Correlations of Nu and Eu for the 10-row tube bundle are presented.

Natural convection and radiation heat transfer of an externally-finned tube vertically placed in a chamber

A three-dimensional numerical study was made to investigate effects of fin angle, fin surface emissivity, and tube wall temperature on heat transfer enhancement for a longitudinal externally-finned tube placed vertically in a small chamber. The numerical model was first validated through comparison with experimental measurements and the appropriateness of general boundary conditions was examined. The numerical results show that the mean Nusselt number increases with Rayleigh number for all the fin angles investigated. The maximum heat transfer rate per mass occurs when the fin angle is about 60° for fin surface emissivity between 0.7 and 0.8 and 55° when the surface emissivity increases to 0.9. With increasing tube wall temperature, both the natural convection and radiation heat transfer are enhanced, but the fraction of radiation heat transfer decreases in the temperature range studied. Radiation fraction increases with increasing fin surface emissivity. Both convection and radiation heat transfer modes are important.

Solar magnetic flux tube simulations with time-dependent ionization

In the present work we expand the study of time-dependent ionization previously identified to be of pivotal importance for acoustic waves in solar magnetic flux tube simulations. We focus on longitudinal tube waves (LTW) known to be an important heating agent of solar magnetic regions. Our models also consider new results of wave energy generation as well as an updated determination of the mixing length of convection now identified as 1.8 scale heights in the upper solar convective layers. We present 1-D wave simulations for the solar chromosphere by studying tubes of different spreading as function of height aimed at representing tubes in environments of different magnetic filling factors. Multi-level radiative transfer has been applied to correctly represent the total chromospheric emission function. The effects of time-dependent ionization are significant in all models studied. They are most pronounced behind strong shocks and in low density regions, i.e., the middle and high chromosphere. Concerning our models of different tube spreading, we attained pronounced differences between the various types of models, which were largely initiated by different degrees of dilution of the wave energy flux as well as the density structure partially shaped by strong shocks, if existing. Models showing a quasi-steady rise of temperature with height are obtained via monochromatic waves akin to previous acoustic simulations. However, longitudinal flux tube waves are identified as insufficient to heat the solar transition region and corona in agreement with previous studies.

Microstructural change of a 9Cr steel longitudinal welded tube under internal pressure creep loading

In this study, the microstructure of a base metal, and the heat-affected zone (HAZ) and weld metal of a 9Cr steel longitudinal welded tube with various internal pressure creep damage levels was studied by transmission electron microscopy and energy dispersive X-ray spectroscopy. Each portion of the steel weldment clearly changed with damage level. The present results indicate that the recovery of the microstructure is faster in HAZ than in other portions, and thus creep deformation preferentially occurs in HAZ. Moreover, it was revealed that stress accelerates the growth of M23C6 precipitates as well as the reduction of dislocation density, consequently promoting recovery. It was also confirmed that the so-called type IV failure is reasonably explained by precipitate strengthening.

Parametric study and optimization of louver finned-tube heat exchangers by Taguchi method

Abstract A serial studies on the effects of fin pitch, fin collar outside diameter, transverse tube pitch, longitudinal tube pitch, number of longitudinal tube rows, louver height, louver angle, fin thickness and louver pitch on fin performance of louver finned-tube heat exchanger is investigated by numerical method. The parameters of louver finned-tube heat exchangers are optimized by the Taguchi method. Eighteen kinds of models are made by compounding levels on each factor, and the heat transfer and flow characteristics of each model are analyzed. The results show that fin collar outside diameter, transverse tube pitch and fin pitch, are the main factors that influence significantly the thermal hydraulic performance of the heat exchanger. Therefore, these three factors are considered as the most important factors for an optimum design of a heat exchanger. The optimal conditions are obtained, and the results are also verified by analytical methods.

Air-side heat transfer and pressure drop in spiral wire-on-tube condensers

The air-side thermal-hydraulic performance of spiral wire-on-tube condensers is investigated experimentally in this paper. Sixteen prototypes have been manufactured and tested in an open-loop wind tunnel calorimeter. The influence of the following geometric parameters has been evaluated: the number of tube passes, the radial and longitudinal tube spacings and the wire spacing. Measurements of the air-side thermal conductance and pressure drop were carried out for air flow rates ranging from 70 to 220 m3 h−1. The data were correlated using empirical relationships for the Colburn j-factor and the Darcy friction factor. The agreement with the experimental data presented RMS deviations of 0.9% for the air-side heat transfer and 1.3% for the frictional pressure drop dimensionless parameters. A quantitative analysis based on the core volume goodness factors for heat transfer and pressure drop is presented to provide the most viable configuration from the point of view of application in a refrigerator.

Experimental Investigation of Heat Transfer and Flowing Resistance for Air Flow Cross over Spiral Finned Tube Heat Exchanger

In order to study heat transfer and flowing resistance characteristics for air flow cross over spiral finned tube heat exchanger, 13 spiral finned tube bundles heat exchangers are experimental investigated, the specimens have constant outer diameter of base tube (d=32mm), different fin pitch (t/d=0.22∼0.5), fin height (h/d=0.22∼0.5), transverse tube pitch (S1/d=2∼3.3125), longitudinal tube pitch (S2/d=2∼3.3125), the experiments are done within the range of fluid flowing Re number (Re=5×103∼5.5×104). Experiments obtain heat transfer Nusselt number correlation and flowing resistance Euler number correlation with fluid flowing Re number, fin pitch t, fin height h, transverse tube pitch S1, and longitudinal tube pitch S2.

Experimental investigation of heat transfer and pressure drop in serrated finned tube banks with staggered layouts

Abstract An experimental study was performed to investigate the heat transfer and pressure drop characteristics of serrated finned tube banks with staggered layouts. The influences of varied fin densities, transversal tube spacing and longitudinal tube spacing were presented. For a constant fin height, an increase in the fin density resulted in an increase in the Euler number, and a gradual decrease in the Nusselt number was observed as the Reynolds number increased. An increase in the transversal tube spacing corresponded to a significant reduction in the Euler number, whereas the Nusselt number essentially remained unchanged. The longitudinal tube spacing had an insignificant effect on the Nusselt and Euler numbers, and the optimum ratio of the transversal tube spacing to longitudinal tube spacing increased with an increase in the transversal tube spacing. Scaling of the tube spacing had little effect on the Nusselt number but had a significant influence on the Euler number. Finally, predictive correlations for Nusselt and Euler numbers were developed based on the data presented here, and comparisons between the test data from this paper and the predictive values from previously published correlations were performed.

Metal foam heat exchangers for heat transfer augmentation from a tube bank

A numerical study has been conducted to examine the heat transfer from a metal foam-wrapped tube bundle. Effects of key parameters, including the free stream velocity, longitudinal and transversal tube pitch, metal foam thickness and characteristics of the foam (such as porosity, permeability, and form drag coefficient) on heat and fluid flow are examined. It can be observed that the performance of the metal foam heat exchangers, measured in terms of area goodness factor, can noticeably be better than that of the conventional design of finned-tube heat exchangers. It is also found that even a very thin layer of metal foam, when wrapped around a bare tube bundle, can significantly improve the area goodness factor. Finally, it is shown that while friction factor is more sensitive to the metal foam permeability than its porosity, the converse is true when it comes to the Colburn factor.

Performance of longitudinal and volumetric tube current modulation in a 64-slice CT with different choices of acquisition and reconstruction parameters

Aim of the study was to evaluate the performance of a tube current modulation (TCM) system (SUREExposure 3D).On a 64 detector-row CT scanner (Aquilion 64, Toshiba), performance of fixed tube current, longitudinal TCM, and volumetric TCM acquisitions were assessed. A homogeneous cone-shaped phantom and an anthropomorphic phantom were used. Tube current and noise profiles were quantitatively analysed by box and whisker plots when phantom size, acquisition, and reconstruction parameters were varied.At similar median noise, fixed tube current scanning showed a noise range of 16.8–38.3 HU, while longitudinal TCM showed a range of 19.4–31.4 HU and volumetric TCM showed an even lower range of 20.7–28.7 HU. When acquisitions resulting in similar image quality (noise) were compared, the use of volumetric compared to longitudinal TCM resulted in a variable radiation dose reduction up to 6.6%.In conclusion, SUREExposure 3D resulted in more uniform image quality at a lower dose. Volumetric TCM shows improved results over longitudinal TCM.

Modelling of creep damage in high temperature component life assessment

Creep damage gradually proceeds in high temperature components in thermal power plants featuring creep void nucleation and growth. Accurate prediction of the creep damage is necessary for reliable operation and life extension of these components. Microstructural changes of a 1%CrMoV steel and a longitudinal welded tube of 9%Cr steel during creep were observed with creep test interrupted specimens by means of SEM. It was suggested that void growth is controlled mainly by a diffusion mechanism affected by matrix creep deformation and constraint of surrounding grains. Based on microstructural observation and theoretical considerations, a void initiation and growth model was proposed, and a void simulation program incorporating the void initiation and growth model was developed. It was demonstrated that void initiation and growth behaviour in homogenous materials as well as in heat affected zone in a welded tube was quantitatively predicted by the void simulation program. This suggests that local damage evolution in components may be predicted by the proposed method.

CT Dose Index and Patient Dose: They AreNotthe Same Thing

Estimates of individual patient risk, and epidemiologic studies assessing potential late effects, must use patient size–specific dose estimates—they cannot use only scanner output (volume CT dose index or dose-length product).

Blood flow dynamics of one cardiac cycle and relationship to mechanotransduction and trabeculation during heart looping.

Analyses of form-function relationships during heart looping are directly related to technological advances. Recent advances in four-dimensional optical coherence tomography (OCT) permit observations of cardiac dynamics at high-speed acquisition rates and high resolution. Real-time observation of the avian stage 13 looping heart reveals that interactions between the endocardial and myocardial compartments are more complex than previously depicted. Here we applied four-dimensional OCT to elucidate the relationships of the endocardium, myocardium, and cardiac jelly compartments in a single cardiac cycle during looping. Six cardiac levels along the longitudinal heart tube were each analyzed at 15 time points from diastole to systole. Using image analyses, the organization of mechanotransducing molecules, fibronectin, tenascin C, α-tubulin, and nonmuscle myosin II was correlated with specific cardiac regions defined by OCT data. Optical coherence microscopy helped to visualize details of cardiac architectural development in the embryonic mouse heart. Throughout the cardiac cycle, the endocardium was consistently oriented between the midline of the ventral floor of the foregut and the outer curvature of the myocardial wall, with multiple endocardial folds allowing high-volume capacities during filling. The cardiac area fractional shortening is much higher than previously published. The in vivo profile captured by OCT revealed an interaction of the looping heart with the extra-embryonic splanchnopleural membrane providing outside-in information. In summary, the combined dynamic and imaging data show the developing structural capacity to accommodate increasing flow and the mechanotransducing networks that organize to effectively facilitate formation of the trabeculated four-chambered heart.

Generation of longitudinal flux tube waves in theoretical main-sequence stars: effects of model parameters

Aims. We compute the wave energy fluxes carried by longitudinal tube waves along vertically oriented thin magnetic fluxes tubes embedded in the atmospheres of theoretical main-sequence stars based on stellar parameters deduced by Kurucz and Gray. In addition, we present a fitting formula for the wave energy flux based on the governing stellar and magnetic parameters.

A numerical investigation of the geometric effects on the performance of plate finned-tube heat exchanger

This study numerically examines the geometric parameters on the performance of a two-row fin-and-tube heat exchanger. Effects of fin pitch, tube pitch, fin thickness, and tube diameter are termed with. The simulation indicates that the performance, in terms of Q/Δ P and COP, increases with longitudinal tube pitch or with transverse tube pitch, and it decreases with larger tube diameter or fin thickness. An optimum value for Q/Δ P occurs at a 6–8 fpi at a fixed flow rate condition. There is not much difference in choosing the index of Q/Δ P or COP under fixed flow rate condition. However, when the simulation are performed with the actual axial fan whose P– Q curve being implemented. It is found that Q/Δ P peaks at 12 fpi while COP peaks at 16 fpi.

Damage Evolution and Life Prediction of a P91 Longitudinal Welded Tube Under Internal Pressure Creep

The clarification of creep damage mechanism and the establishment of remaining life prediction methods of longitudinal welded piping of P91 steel are important subjects to maintain a reliable operation of boilers in thermal power plants. Internal pressure creep tests were conducted on P91 steel longitudinal welded tubes to characterize the evolution of creep damage with time and to evaluate a life prediction method. Interrupted creep tests were performed for damage observation in addition to rupture tests. Three dimensional finite element creep analyses of the longitudinal welded tube specimens were conducted to identify the stress and creep strain distributions within the specimen during creep. Failure occurred at a heat affected zone (HAZ) without a significant macroscopic deformation. It was found that the initiation of creep voids had concentrated at the midthickness region in the HAZ rather than in the surface. The creep analysis results indicated that the triaxial tensile stress yielded at the midthickness region in the HAZ due to difference of creep deformation property among the base metal, the HAZ, and the weld metal. It was suggested that the triaxial stress state caused acceleration of the creep damage evolution in the HAZ, resulting in internal failure of the tube specimens. A rupture time prediction method of the welded tube is proposed based on the maximum principal stress and the triaxial stress factor in the HAZ. The void growth behavior in the HAZ was well predicted by the previously proposed void growth simulation method by introducing a void initiation function to the method.

Arcyria monticola from Hokkaido, northern Japan: its habit, detailed morphology, and comparison with Arcyria imperialis

Arcyria monticola was found in Hokkaido, northern Japan. This is the first record of its locality, except for the type locality. The linearly expanding capillitium with many coils characterizing A. monticola consists of very elastic longitudinal tubes, short lateral tubes, and many small intricate nets. A longitudinal tube coils around a small intricate net, which forms a core. In contrast, A. imperialis, the other known Arcyria species with a linearly expanding capillitium, has neither short lateral tubes nor nets. The microscopic character of the A. imperialis capillitium is also totally different from that of A. monticola.

Optimization of heat exchangers with vortex-generator fin by Taguchi method

A comparative study of effects of attack angle, length of vortex generator, height of vortex generator, fin material, fin thickness, fin pitch and tube pitch on fin performance of vortex-generator fin-and-tube heat exchanger is conducted by numerical method. The parameters of vortex-generator fin-and-tube heat exchangers are optimized by the Taguchi method. Eighteen kinds of models are made by compounding levels on each factor, and the heat transfer and flow characteristics of each model are analyzed. The results show that these six factors (fin pitch, longitudinal tube pitch, transverse tube pitch, length of vortex generator, height of vortex generator, and attack angle of vortex generator) have great influences on the JF-factor. The fin material and fin thickness have trifling effects on the JF-factor. The two optimal conditions (A1B3C3D2E1F2G1H3 and A2B2C2D3E1F2G1H3) are acquired, and the reproducibility of the results is verified by two analytical results.